It is known to compress the air which is fed to an internal combustion engine for the combustion in the cylinders by way of exhaust gas turbochargers which are driven by the exhaust gas energy of the internal combustion engine or by way of mechanical superchargers. A performance increase of the internal combustion engine is achieved as a result. If mechanical superchargers are used, they can be driven directly by the internal combustion engine, for example via a belt drive, or they can be driven by an auxiliary motor, for example an electric motor.
If the air which is fed to the internal combustion engine is compressed by an exhaust gas turbocharger, it is known that, in the case of sudden load requests, for example during the transition from the part load range into the full load range during acceleration of a vehicle which is driven by way of the internal combustion engine, the exhaust gas turbocharger is not capable of providing the charge air quantity which is required to provide the torque which is requested from the internal combustion engine. This effect is frequently also called “turbo lag”. This is caused by the fact that the existing exhaust gas energy is not sufficient to drive the turbocharger in such a way that the desired boost pressure can be provided. Inertia of the turbocharger also leads to a delay in the build up of boost pressure occurring.
These problems are solved firstly by structural changes of the turbocharger, for example by turbochargers with a variable turbine geometry. Another solution which is known in the prior art consists of providing an electric motor on the shaft of the turbocharger (what is known as an “electrically assisted exhaust gas turbocharger”).
A third option consists of providing a mechanically driven supercharger in addition to the exhaust gas turbocharger, which supercharger provides boost pressure only when, on account of sudden load requests, the exhaust gas turbocharger is not capable of providing the required boost pressure immediately, that is to say is subject to turbo lag. Here, the mechanical supercharger can in principle be driven by the internal combustion engine, as is also predominantly the case in mechanical superchargers which are used exclusively to supercharge the internal combustion engine. Since, here, the mechanical supercharger assists merely the exhaust gas turbocharger at certain operating points, however, assisting mechanical superchargers of this type are predominantly driven not by the internal combustion engine itself, but rather by a dedicated auxiliary motor.
In order, during this turbo lag, for it to be possible for the missing, required boost pressure which is not provided by the exhaust gas turbocharger to be provided by the mechanically driven additional supercharger, the latter has to have a very short response time. If the mechanical supercharger is driven only when it is required for the provision of the charge air, delays occur as a result of the running up of the mechanical supercharger. For running up, the superchargers which currently respond most quickly require a start up time in the order of magnitude of from 350 to 400 ms. If the mechanical supercharger is driven by an electric motor, a very high current consumption by the electric motor occurs during this running up. However, a current consumption of this type is not permissible in the case of conventional 12 volt vehicle on-board electrical systems, since it can lead to considerable voltage dips. It lies in the order of magnitude of 350 A starting current and 250 A steady current in the case of an output power in the order of magnitude of 2.6 kW.